A process for the quality assessment and color control in the offset printing known, for example, from U.S. Pat. No. 5,182,721 and U.S. Pat. No. 6,041,708 consists in that a color measuring strip is printed together with the actual image, which includes a number of defined color control elements with known set values. The color values of the co-printed color control elements are measured and compared with the matching set values. The color differences determined thereby serve as a measure of quality and are also used for the control of the inking in the printing machine, whereby the control is carried out such that the total color difference is minimized. This minimization of the total color difference is thereby limited to the printed colors in the color measuring strip. However, those colors are not always representative of the relevant colors of the image motive so that the achieved color quality of the print is in many cases not optimal.
A further developed process, generally referred to as in-image-measurement and, for example, described in U.S. Pat. No. 5,957,049, is based on that the printed image itself is colorimetrically measured and used for the quality assessment and color control. The printed image is thereby scanned with a local image resolution of, for example, 1 mm * 1 mm. This takes place, for example, in an image scanner or in an electronic camera. The measured data (color values of the individual pixels) are compared with the corresponding measured data of a reference print or with the image data from the printing precursor. The color differences produced thereby again serve as quality measure and/or for the control of the inking, whereby the mean color error is minimized over the whole printed image. This process is technically very laborious and computing time intensive, whereby a special problem is the locally exact overlapping of the reference image data with the measured data of the printed image to be tested.